Cathode ray storage tube for displaying stored and non-stored displays in different colors



Nov. 8, i966 .L BRAMLEY ETAL 3,284,554

CATHODE RAY STORAGE TUBE FOR DISPLAYING STORED AND NON-STORED DISPLAYSIN DIFFERENT COLORS Filed Jan. z, 1963 AND EFLECTION CURRENT GENERATOR lI NVE NTORS.

1 O Norman H. Lehrer, l

Z55 Jenny Bromley, N- B United States Patent Ofice 3,284,654 PatentedNov. 8, 1966 3,284,654 CATHODE RAY STORAGE TUBE FOR DISPLAY- ING STOREDAND NON-STORE!) DISPLAYS IN DIFFERENT COLORS Jenny Bramley, FallsChurch, Va., and Norman H. Lehrer, Pacific Palisades, Calif., assignorsto Hughes Aircraft Company, Culver City, Calif., a corporation ofDelaware l Filed Jan. 2, 1963, Ser. No. 249,018 1 Claim. (Cl. 313-92)This invention relates to visual display storage tubes. Moreparticularly, the invention relates to color display storage tubeswhereby stored displays or any portion thereof may be selectively erasedand both stored and non-stored displays may be simultaneously presentedin different colors.

Heretofore, the most practical and useful visual display storage tubeshave utilized storage targets which have operated primarily, if notentirely, on the phenomenon of secondary electron emission from thestorage surface. However, in the co-pending application of Norman H.Lehrer, Serial No. 59,590 filed September 30, 1960, now U.S. Patent No.3,086,139 issued April 16, 1963, a novel cathode ray storage tubeutilizing the phenomenon -of bombardment induced conductivity isdescribed. As explained therein such a storage tube permits selectiveerasure of stored displays or any portion thereof as well as thesimultaneous display of both stored and non-stored information.

In general, this tube employes a target comprising a conductive supportmember having a thin film thereon of cubic zinc sulde. The cubic zincsulfide surface is responsive to the energy level of an electron beamimpinging thereon whereby at one beam energy level the principal effectis secondary electron emission greater than unity while at a differentbeam energy level the principal effect is bombardment inducedconductivity. Thus, while at the first beam energy level somebombardment induced conductivity may occur, it is completely dominatedor overridden by the secondary electron emission phenomenon. Such astorage target is capable of being charged in opposite electricalsenses. by selectively utilizing these two phenomena. It is thuspossible, by switching the electron beam energy level, to write or storeinformation on the lstorage surface by one phenomenon and to erase byselective scanning any portion of the stored information by the secondphenomenon. Thus, for example, a normally negatively charged storagetarget may be driven selectively positive by secondary electron emissionin accordance with information to be stored by scanning the target withan electron beam of 2.5 kv. This permits relatively low energy floodelectrons to penetrate the storl l age target and excite the phosphorscreen into luminescence in accordance with the charge pattern thereon.Bombardment of these positively charged portions with an electron beamof 7.0 kv., for example, induces those portions to become conductive sothat they return to or assume the normally negative potential of thesto-rage target. It is also possible by utilizing an electron beamhaving an intermediate energy level (i.e., 4.5 kv.) to cause the beam topass through the storage target without altering the stored pattern orother potential conditions of the target. Thus stored information may bedisplayed simultaneously with non-stored or live information. Thepresent invention is directed toward providing means for readilydistinguishing stored information from non-stored information.

One object of the present invention is to provide an improved cathoderay storage tube.

Another object of the invention is to provide an improved cathode raycolor storage tube utilizing the phenomenon of bombardment inducedconductivity and being capable of providing stored and non-storeddisplays in different colors.

These and other objects and advantages of the invention are realized byproviding a multi-layer phosphor viewing screen in a bombardment inducedconductivity storage tube and controlling the excitation of particularphosphor layer in accordance with electron velocity. Thus, in the typeof tube described in the aforementioned co-pending application, onephosphor layer would be excited to produce green light, for example, bythe action of relatively low velocity flood electrons for storeddisplays, and a second phosphor layer would be excited to produce redlight, for example, by the action of the higher velocity electrons ofthe non-storage electron beam of 4.5 kv.

The invention will be described in greater detail by reference to thefollowing drawings in which:

FIGURE l is a partially cross-sectional and partially schematic view ofa cathode ray storage tube employing a multi-layer phosphor viewingscreen according to the present invention; and

FIGURE 2 is a cross-sectional elevational view of an arrangement of amulti-layer phosphor viewing screen according to the present invention.

Referring now to FIGURE 1 a half-tone visual display cathode ray tube 12is shown according to the present invention. The tube 12 comprises anevacuated envelope formed by a comparatively large cylindrical section14 and a narrower neck portion 16 communicating therewith at one sidethereof (hereinafter referred to the neck or gun side). The neck section16 may be disposed, as shown, at an angle with respect to the mainlongitudinal axis of the larger cylindrical section 14. The side of thelarge cylindrical section 14 opposite the neck side comprises aface-plate 18 over the inner surface of which is a multi-layer phosphortarget which will be described in greater detail hereinafter. Adjacentand coextensive with the face-plate 18 is a storage target 2 asdescribed in the vaforementioned co-pending application. The `storagetarget 2 may comprise a nickel mesh, which may be electroformed, havingdisposed on one side thereof a thin layer or film of cubic zinc sulfidewhich has both secondary electron emission and bombardment inducedconductivity properties. The mesh may have from to 400 meshes per inch,preferably 250 meshes per inch, and a thickness of about 1 to 2 mils.Such a mesh with a pitch of 250 meshes per inch will have an overalltransparency'of about 60%. The cubic zinc sulfide layer is disposedcoextensive with the meshes of the screen and may be about one to twomicrons thick, for example.

It may also be desirable and preferred to provide a supplementary layerof secondary electron emissive material over the cubic zinc sulfide lmin order to enhance the secondary electron emission characteristics ofthe storage target. Thus, for example, a thin film of magnesium fluoridemay be evaporated onto and over the layer of cubic zinc sulfide. Thefilm of magnesium fluoride may be about 500 Angstroms thick, forexample. This supplemental secondary electron emissive layer should bethin enough to allow a high energy level (i.e., 7 kv.) electron beam topenetrate therethrough to the cubic zinc sulfide layer so as to raiseelectrons therein to the conductive energy level and thick enough toprovide high secondary electron emission when bombarded by a relativelylow energy level (i.e., 2.5 kv.) electron beam.

Continuing to proceed from the viewing screen end of the tube toward thegun section, a collector grid 24 is disposed adjacent and -coextensivewith the storage target 2. The collector grid 24 comprises a conductivescreen supported about its periphery by an annular ring 26. Thetransparency of this screen is preferably of the order ofY 80%; thefunction of the grid 24` is to collect secondary electrons emitted fromthe storage target 2. Adjacent -the collector grid 24 is a collimatingelectrode 28 in the form of .a cylindrical can, the purpose of which isto collimate ood or viewing electrons from the flood gun 30 which isdisposed at the gun side of the tube section 14.

The ood gun 30, which may be on the longitudinal axis of the largercylindrical portion 14 of the tube 12, comprises a cathode 32 and anintensity electrode 34 which encloses the cathode 32 except for a smallaperture 36 disposed over the central portion of the cathode 32. Anannular electrode 33 is disposed adjacent the intensity electrode 34 andcoaxially with respect to the longitudinal axis of the tube 12 whichalso passes through the center of the aperture 36 in the intensityelectrode 38.

The neck portion 16 of the tube 12 houses an electron gun 40 which maybe of conventional construction. The gun 40 comprises a cathode 42, anintensity electrode grid 44, and a cylindrical beam-forming section 46.

An equipotential region is maintained throughout the neck portion 16 ofthe larger cylindrical section 14 of the tube 12 by means of aconductive layer 48 which A may be coated over the interior surfaces ofthe tube as shown. During operation, a potential of about 5 voltspositive may be maintained on this conductive layer.

The viewing screen according to the present invention comprises a firstphosphor layer 22 disposed on the internal surface of the tubeface-plate 18, for example, a second phosphor layer 22 disposed over thefirst phosphor layer, and an electrically conductive, electrontransparent, optically .reflecting coating 23 disposed over the secondphosphor layer. The reflective coating 23 may comprise a thin film ofaluminum, for example, the application and use of which are well knownin the cathode ray tube art. These phosphor layers may be either of thesettled type with a microcrystalline structure or ofthe evaporated type,which are transparent. The latter embodiment may be preferable from thepoint of view of improved resolution. The first phosphor layer 22 shouldbe of a phosphor material which is excitable by relatively high velocityelectrons, as for exam-ple the electrons in the nonstorage electronbeam. A suitable phosphor for this purpose is magnesium fiuoride, forexample.` This phosphor emits red light upon excitation. The secondphosphor layer 22 should be of a phosphor material which is excitableprincipally by only relatively low velocity electrons, as for examplethe flood or viewing electrons. This second phosphor layer should alsobe capable of emitting a different color light in comparison with thelight emitted by the first phosphor layer. A suitable phosphor for thesecond phosphor layer is zinc orthosilicate which emits green light uponexcitation. The principle of color selection by the use of such amulti-layer viewing screen is based on the fact that for a givenphosphor layer thickness the light emitted does not continuouslyincrease with beam energy but reaches a maximum and then declines. Inaddition, the velocity of the impinging electrons will determine whichlayer the electrons lose most of their velocity in and produce lightmost efiiciently. For the electron velocities involved in a tube asdescribed herein the thickness of the phosphor layers may be about onemicron each.

In FIGURE 2 an alternate phosphor layer arrangement is shown, theprincipal difference being the incorporation of an additional layer 23between the phosphor layers 22, 22. This layer 23 may be of zirconiumsilicate and its purpose is to prevent impurity in the colors presentedby the various electron-beam energy levels. For example, in the case ofthe electrons from the flood gun which strike the viewing screen with 6kv. energy, if some of these electrons completely penetrate the irstphosphor layer, then they could cause the second phosphor layer toluminesce resulting in color impurity. By the addition of thenon-luminescing layer (zirconium silicate) between the two phosphorlayers, these electrons are absorbed and such color contamination isprevented.

The storage target characteristic is such that with a beam energy ofabout l kv., charging (which means charging the potential of the storagesurface) is at a maximum and is almost entirely due to -the secondaryelectron emission phenomenon, any charging due to bombardment inducedconductivity being negligible and rather completely overridden by thesecondary emission eiiect. Thus with a primary beam energy of from 1 to4.5 kv. the storage target is charged positively by the secondaryemission phenomenon.

At about 4.5 kv. secondary emission still occurs but bombardment inducedconductivity effects will have increased to the point where bothphenomena charge the storage surface in equal but opposite electricalsenses, hence the storage surface potential will be undisturbed when thestorage target is bombarded by a beam of about 4.5 kv. With beamenergies greater than about 4.5 kv. the bombardment induced conductivityeffect increases further and rather completely overrides the secondaryemission effect which continues to diminish. The net charging effect onthe storage surface hence is to -drive it negatively to an equilibriumpotential by the bombardment induced conductivity effect.

It will thus be appreciated that the storage target of the presentinvention utilizes two Aphenomena to produce charging effects inopposite electrical senses which effects may be balanced so as to resultin no net charging effect in either electrical sense. This is possiblebecause there is a continuous range of electron beam energy levels whereboth secondary electron emission and bombardment induced conductivityoccur and because at different portions of this range either of thesephenomena can be made dominant or the two phenomena can be balanced. Thecapability of balancing these two phenomena is of utmost significancewhere it is desired to provide a storage target which can be writtenthrough to present direct or live information without altering thepotentials on the storage target. Thus if these two phenomena did notoverlap over a continuous range of electron -beam energy levels, onecould still store by one phenomenon (i.e., secondary emission) and eraseby the other (i.e., bombardment induced conductivity) simply byswitching the 'beam energy levels but there would be no energy levelwhere one could write through since changing the beam energy level onlywould result in establishing one of the two phenomena as the dominantone. The charging speed of the target is at a maximum for beam energiesof less than about 2 kv. and more than about 7 kv. It is at a mlnimumfor a beam energy of about 4.5 kv. Because greater display resolutioncan be obtained with a pnimary beam energy of 2 to 3 kilovolts, chargingby secondary emission is preferably accomplished by utilizing a beam ofabout 2.5 kv. rather than 1 kv. or less at a slight sacmfice in chargingspeed. Charging by bomba-rdment induced conductivity is preferablyaccomplished by utilizlng a beam of about 7 kv. in order to achieve acharging speed comparable to that of secondary emission charging whichis about 50,000 to 100,000 inch-volts per second. Beam energies ofgreater than 7 kv. begin to introduce serious problems of electricalinsulation.

Operation of a selective erasure storage tube may be accomplished asfollows. A potential of about 9 volts negative relative to ground isapplied to the nickel mesh support of the storage target. The flood orviewing gun cathode 32 may be maintained at ground potential while theintensity electrode 34 and the annular electrode 38 may be maintained,respectively, at potentials of about 20 volts negative and 100 voltspositive with respect to ground. Under these cirmustances ood electronsfrom the gun 30 will be prevented from penetrating the storage target 2(because of the 9-volt negative potential thereon).

Hence the iiood or viewing electrons cannot reach the viewing screen andexcite it into luminescence. This is the initial dark condition of thetube.

To store and display information, the storage target 2 is scanned by anelectron beam of elemental cross-sectional area having an energy levelof about 2.5 kilovolts. This beam may be generated by means of theelectron gun 40 in the neck portion 16 of the tu'be. The cathode 42 ofthis gun may be maintained at a potential of about 2000 volts negativewith respect to ground while the intensity grid 44 may be at a potentialof about 75 volts negative with respect to the potential of the cathode42. The electron beam produced by this gun is modulated and scanned inaccordance with information-representative signals derived and appliedby conventional techniques. The beam is deflected horizontally andvertically electromagnetically, as shown, by means of the deflectionyoke S2 which'is positioned around the neck 16 of the tube.

Areas of the storage target 2 impinged by the 2.5 kv. beam in accordancewith the information to be displayed are charged positively due to theemission of electrons therefrom which are collected Iby the collectorgrid 24 which may be maintained at a potential of 120 volts positiveWith respect to ground in order to accomplish this function. Viewing orflood electrons from the flood gun 30 may then pass through the storagetarget 2 at these areas of positive potential and are then acceleratedto impinge upon the phosphor layer 22 of the viewing screen `by means ofa potential of about 6,000 volts positive with respect to ground whichmay be maintained on the aluminum lm 20 of the viewing screen. In thismanner the information is displayed in green light and the display maybe maintained and viewed as long as desired.

Non-stored or live information may also be simultaneously displayed byswitching the potential of the cathode 42 of the charging gun 40 toabout 4.5 kilovolts. As explained previously a beam of this energy leveldoes not produce any change in the potential of the storage surface.Hence, the -beam passes through the storage target 2 without alteringthe potential of either positively or negatively charged port-ions, andis accelerated to a velocity of l0 plus kv. to penetrate into thephosphor layer 22 where most of the velocity thereof is dissipated toresult in a display of live or non-stored information in red light.

Stored potentials on the storage ta-rget 2 may be selectively erased byswitching the potential of the cathode 42 of the charging gun 40 toabout 7.0 kilovolts and scanning the storage target with the beam ofthis energy level in accordance with signals representing theinformation to be erased. The impingement of a beam of 7.0 kv. onportions of the storage target results in these portions being chargednegatively to about the potential of the nickel support mesh 4 (-9volts) by means of the phenomenon of bombardment induced conductivity,as explained previously.

While the storage tube shown in FIGURE 1 and described so far herein hasbut one charging electron gun Whose cathode potential is switched to=provide beams of different energy levels (2.5 kv., 4.5 kv., and 7.0kv.) so as to permit storing, writing-through, and erasing selectively,additional electron guns may be provided to perform these functionsindependently whereby storing, writing-through, and erasure can beaccomplished simultaneously.

There thus has been described a new and improved cathode ray storagetube utilizing the phenomenon of bombardment induced conductivityeffectively and in a practical manner which permits selective erasure ofstored information as well as the presentation of live informationsimultaneously with the display of stored information in different orcontrasting colors.

What is claimed is:

A half-tone visual display storage tube comprising:

(a) a storage target including a conductive support member having alayer of bombardment induced conductivity material on at least a portionof one side thereof and exhibiting both 'bombardment inducedconductivity and secondary electron emission over a continuous range ofelectron beam energy levels whereby said storage target may be chargedin substantially equal and opposite electrical senses at a predeterminedportion of said range;

(b) a first electron source for directing flood electrons uniformly oversaid storage target;

(e) a second electron source for producing a scanning electron beamhaving an energy level corresponding to said predetermined portion ofsaid range of electron energy levels;

(d) and a viewing target disposed adjacent said storage target on theside thereof opposite the said one side and having phosphor meansadapted to produce, respectively, light of different colors in responseto impingement thereof by said flood electrons and -by electrons in saidscanning beam.

References Cited by the Examiner UNITED STATES PATENTS JAMES W.LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, JAMES W. LAWRENCE,

V. LAFRANCHI, Assistant Examiners.

